The whole point of demanding that cars make some sound is so that they are identified as cars. Custom sounds as in custom ringtones would totally thwart that concept. What EVs need to sound like is those early inverter drive forklift trucks. A sound like no other, not obtrusive but a bit scary

It's bad enough having to listen to all the ridiculous ringtones at work. I surely wouldn't want to hear a constant barrage of random sounds from vehicles. There would always be the small percentage of people who would have to get stupid and try spoken phrases, sirens, chickens, etc. As long as there are humans involved, it has to be strictly controlled.

I'd start here... (http://www.nhtsa.gov/About+NHTSA/Press+Releases/DOT+Proposes+New+Minimum+Sound+Requirements+for+Hybrid+and+Electric+Vehicles)... There is at least a contact atop the release.
But more likely is the Federal Register: https://www.federalregister.gov/.
Let us know what you find out!